Method and mechanism for indicating mold friction in a continuous-casting machine

ABSTRACT

A method and mechanism for determining frictional forces between the mold of a continuous-casting machine and the casting itself. During a casting operation the mold oscillates up and down, being driven by a motor and cam mechanism. The invention provides a method and means for determining the mold friction by measuring the armature current through the cam drive motor, but compensating for other factors which contribute to the load on the motor, whereby the average magnitude of the armature current through the oscillation cycle is indicative of mold friction only.

United States Patent Slamar METHOD AND MECHANISM FOR INDICATING MOLDFRICTION IN A CONTINUOUS-CASTING MACHINE July s, 1975 3,478,572 11/1969McRae et a1. 73/9 Primary ExaminerJerry W. Myracle Attorney, Agent, orFirmWalter P. Wood ABSTRACT A method and mechanism for determiningfrictional forces between the mold of a continuous-casting machine andthe casting itself. During a casting operation the mold oscillates upand down, being driven by a motor and cam mechanism. The inventionprovides a method and means for determining the mold friction bymeasuring the armature current through the cam drive motor, butcompensating for other factors which contribute to the load on themotor, whereby the average magnitude of the armature current through thescillation cycle is indicative of mold friction only.

Claims, 3 Drawing Figures ileum LEV-EL l ssr POINT l I e msrzn REFERENCE4 I sis/VAL I8 I 3! l A 4 uoum I. LEVEL 51-5407 L IIIDI 4 T00 urns/var/s/smu 1 G2! I0 1: I I 2o 24 12 I fan-rm 1 i l 0 0 32 7' l A: cunRE/vr=1 1' new suPrLr un/r sinrcn 29 mu. cummvr SIGNAL 42 SPEED SIGNAL) crctz'nvalcaran CALIBRATE 7 43 11- 0mm 4444 conpursn new 0L0 rn/crm/v PIP/NTOUT [mum LEI/2'1.

SETPOINT ll l l4 msrsn I REFERENCE SIGNAL I5 M 2 I 3/ I A uaum LEVELSTEAM I9 INDICATOR I REFER- l I I ENLE i SIGNAL I $23 l i .40

l I I l A o o 'causnlvr 1 32 cunnewr new SUPPLY ARM. E f cunns/vr 29SIG/VAL 22 DRIVES FIG. 2.

3,} Hill? SHEET FIG. 3.

I I l I LIMIT 1. IOU/0 L E v51. mm :4 ran I I L LIQUID LEVEL SET POINTCURRENT FIEL D SUPPL Y CYCLE 'IIVDICA TOR DIGITAL compare-n MOLDFRICTION PRINTOUT 3 new R A s 0L A mm n. P M h z. m a E u R 6 0) A A MEML 3 q C A UR 1 3 ARM M 4 m l an 2 9 TEm 3 2 3:

EI'I 1 METHOD AND MECHANISM FOR INDICATING MOLD FRICTION IN ACONTINUOUS-CASTING MACHINE This invention relates to an improved methodand mechanism for determining frictional forces between the mold of acontinuous-casting machine and the casting itself.

In a conventional continuous-casting operation. liquid metal isintroduced continuously to the top of an open-ended, water-cooled,vertically oscillating mold, and a partially solidified casting ofindefinite length emerges from the bottom. As the casting travelsthrough the mold, frictional forces oppose its movement. Excessive moldfriction may lead to defects in the product. or even may causebreak-outs of liquid metal from the solidified shell of the casting.Hence there is a need to monitor the magnitude of the mold friction andto make certain it does not exceed a predetermined limit. Heretofore themagnitude of the mold friction has been determined with load cellsplaced under the mold, or by measuring the load on the drive motor forthe withdrawal rolls which act on the casting below the mold, as shownin Barnard et al US. Pat. No. 3,047,915 and in Osborn US. Pat. No.2,824,346 respectively. Both these arrangements have disadvantages. Itis costly to install load cells under a mold, and not readilyaccomplished if the mold support was not designed originally toaccommodate them. The second method does not give an accuratedetermination, since a casting travels through a roll-rack, whichabsorbs an indeterminate amount of the mold frictional forces. Hence thechange in load on the drive motor is proportional only to the frictionalforces not absorbed by the roll rack.

Conventional practice is to employ a cam mechanism driven by a variablespeed constant-field d-c motor for oscillating the mold, one example ofwhich is shown in Bode Reissue US. Pat. No. 27,469 of common ownership.During its downward travel in the oscillation cycle the mold moves at aspeed about to percent greater than the casting speed. Usually thecasting speed is adjusted so that the level of liquid metal in the moldremains substantially constant. The speed of the cam drive motorcommonly is adjusted with adjustments in the casting speed to maintainthe speed at which the mold moves downwardly at a predetermined ratio tothe casting speed.

A characteristic of a d-c motor of the type used for driving the cams isthat the magnitude of the armature current is proportional to the loadon the motor at any instant. The load of course varies with the varyingmagnitude of torque which the motor develops through the oscillationcycle. Mold friction is only one of several factors which contribute tothe load on the cam drive motor. Other factors include motor losses,mechanical losses in the gearing and other mechanisms, the varying forcerequired to lift and lower the mold through different portions of itsoscillation cycle, and accelerating forces brought about by speedchanges. Since mold friction is a small factor compared with the others,and the lifting and accelerating forces continually vary through theoscillation cycle, direct measurement of the armature current throughthe cam drive motor does not itself afford a meaningful determination ofmold friction.

An object of my invention is to provide an improved method and mechanismfor determining the magnitude of the mold friction, which method andmechanism not only are readily applied to existing continuous-castingmachines, but also afford accurate determinations.

A further object is to provide an improved method and mechanism fordetermining the magnitude of the mold friction in a continuous-castingmachine in which I utilize a measurement of the armature current throughthe cam drive motor, but in which I compensate for the other factorscontributing to the load on this motor.

A further object is to provide an improved method and mechanism foraccomplishing the foregoing object in which I utilize an integratingamplifier or a digital computer to measure the average armature currentto the cam drive motor through a representative number of oscillationcycles, but calibrated to compensate for the effects of factors otherthan mold friction.

In the drawings:

FIG. I is a line diagram of the circuit utilized in one embodiment of myinvention utilizing an integrating amplifier;

FIG. 2 is a graph illustrating the manner in which the armature currentto the cam drive motor varies through an oscillation cycle with themotor driving an empty mold and during a casting operation; and

FIG. 3 is a view similar to FIG. 1, but showing a modified embodimentutilizing a digital computer.

CONVENTIONAL OPERATION FIG. 1 shows schematically a continuous-castingmold 10 from the bottom of which a casting ll emerges. The moldoscillates vertically, commonly at a rate on the order of about onecycle per second, driven by a variable speed d-c motor 12 acting througha cam mechanism 13. A master reference signal automatically controls thecasting speed so that a substantially constant level of liquid metal ismaintained in mold 10. During normal operation the master referencesignal serves also to adjust the speed of motor 12 so that the speed atwhich mold 10 moves downwardly is maintained at a constant ratio to thecasting speed.

MASTER REFERENCE SIGNAL A conventional arrangement of components forproducing a master reference signal is shown schematically within ablock 14.'These components include a liquidlevel indicator 15, aliquid-level set-point potentiometer 16, a speed-adjusting potentiometerl7 and first and second operational amplifiers l8 and 19. Theliquid-level indicator l5 and the set-point potentiometer l6 transmitinput voltage signals to the first operational amplifier 18representative respectively by the actual level in the mold 10 and thedesired level. The first operational amplifier transmits an outputvoltage signal proportional to the algebraic sum of its two inputsignals. The second operational amplifier 19 receives input voltagesignals from the speed-adjusting potentiometer 16 andfrom the firstoperational amplifier 18 representative respectively of the desiredcasting speed and the error in the liquid level. The second operationalamplifier transmits the master reference voltage signal, likewiseproportional to the algebraic sum of its two input signals. The masterreference signal goes to a third operational amplifier 20 used incontrolling the speed of motor 12, and to other operational amplifiers21, 22, etc. used in controlling other drives of the continuous-castingmachine, not shown since they are not 3 involved in the presentinvention. Reference can be made to Milnes US. Pat. No. 3,204,460 ofcommon ownership for a more detailed showing of a suitable liquid levelindicator and an explanation of how a signal representing the liquidlevel in a mold may be used in controlling casting speed.

During normal operation the master reference signal is one of the inputvoltage signals to the third operational amplifier 20. The other inputvoltage signal to the latter is a feedback signal from a tachometer 23which is operatively connected with motor 12. The output voltage signalfrom the third operational amplifier passes through a shunt 24 to acontrol (not shown) for the armature current of motor 12. The armaturecurrent and the load on the motor are directly proportional to thevoltage at the shunt 24.

DETERMINATION OF MOLD FRICTION The present invention provides means fordetermining the magnitude of the mold friction through a representativenumber of oscillation cycles, for example ten to twenty, at any timeduring a casting operation. This determination is based on a measurementof the voltage at the shunt 24. The components of this means are shownschematically within a block 25 and include a conventional integratingamplifier 26, a calibration potentiometer 27, and a voltmeter 28calibrated to read mold friction directly. The present invention makesthis possible by compensating for the other factors which contribute tothe load on motor 12, as hereinafter explained.

COMPENSATION FOR VARYING LIFTING AND LOWERING FORCES FIG. 2 shows theapproximate way in which the armature current to motor 12 varies duringthe oscillation cycle. Curve A is representative of the magnitude of thearmature current when the motor is driving an empty mold up and down.Curve B is representative of the magnitude of the armature currentduring a casting operation. In each instance the magnitude followsapproximately sine wave, reaching maximum values as the mold travelsdownwardly. Curve B lies approximately a uniform distance above curve Aand this distance represents the portion of the current utilized inovercoming mold friction. The integrating amplifier 26 automaticallyaverages the measured magnitude of the armature current and thuscompensates the reading on the voltmeter 28 for the varying forcerequired to lift and lower the mold through different portions of eachoscillation cycle.

COMPENSATION FOR MOTOR LOSSES AND MECHANICAL LOSSES I compensate thereading on voltmeter 28 for motor losses and mechanical losses in thegearing and other mechanisms by adjusting the calibration potentiometer27. I operate the motor 12 through at least a representative number ofoscillation cycles with the mold empty at a predetermined speedapproximately equal to the speed of the motor during a castingoperation. I adjust the potentiometer 27 so that the voltmeter 28 readszero when the motor operates with the mold empty, whereby subsequentvoltmeter readings during a casting operation exclude the portion of thearmature current utilized in overcoming the foregoing losses. I haveobserved these losses vary during a casting operation.

When the casting machine is started after several hours of standingidle, the losses are at their maximum. As a casting operationprogresses, the losses diminish and ultimately reach a steady magnitude.For this reason I prefer to adjust the calibration potentiometer 27 immediately after completion of a casting operation. The adjustment remainsvalid for the next casting operation, provided conditions are similar.

COMPENSATION FOR ACCELERATING FORCES Preferably I also compensate thereading on voltmeter 28 for accelerating forces brought about by speedchanges. The voltage of the master reference signal continuously variesto maintain a constant level of liquid metal in the mold. Each change inthe master reference signal of course changes the voltage at shunt 24and the armature current to motor 12. The change in armature currentproduces an acceleration or deceleration of the mold. Withoutcompensation, such changes would be read on the voltmeter 28 as erraticvariations in the mold friction. I compensate for accelerating forces byutilizing a steady reference signal to control the speed of motor 12 inplace of the master reference signal throughout the oscillation cyclesduring which I determine mold friction. The magnitude of the steadyreference signal approximately equals the magnitude of the masterreference signal at the beginning of this determination. The castingoperation is not appreciably upset if the rate at which the moldoscillates remains constant through a representative number of cyclesneeded to obtain a determination of mold friction.

The components which produce the steady reference signal are shownschematically within a block 29 and include a blocking rectifier 30, apotentiometer 31 and a differential voltmeter 32. The blocking rectifieris connected between the second and third operational amplifiers l9 and20 and between the potentiometer 31 and the third operational amplifier.The voltmeter 32 has a center zero point and is connected between thelines which are at the master reference voltage and the voltagetransmitted by the potentiometer 31 to measure the difference betweenthese voltages. During normal operation of the casting machine, thepotentiometer 31 is adjusted so that the master reference voltageexceeds the voltage transmitted by the potentiometer, the voltmeter 32shows a positive voltage, and the blocking rectifier 30 prevents anyvoltage from the potentiometer from reaching the third operationalamplifier 20. Whenever I wish to determine mold friction, I adjust thepotentiometer 31 so that its voltage slightly exceeds the masterreference voltage whereby the potentiometer transmits a steady referencesignal which is approximately equal to the master reference signal atthe moment. The blocking rectifier now prevents the master referencesignal from reaching the third operational amplifier 20, and the steadyreference signal takes over control of the armature current throughmotor 12. After I observe mold friction through a representative numberof oscillation cycles, I return the potentiometer 31 to its originalsetting and resume normal operation.

MODIFICATION USING DIGITAL COMPUTER FIG. 3 shows a modification usefulfor casting installations in where there is a digital computeravailable. Some casting installations employ a computer for otherpurposes. such as controlling the volume of water applied to the castingto cool it and promote solidification. In such installations thecomputer conveniently can be used to obtain a print-out of the moldfriction in place of the integrating amplifier shown in FIG. 1. If theinstallation does not already include a digital computer, the simplerembodiment of FIG. 1 of course is preferred. Except as specificallydescribed, the embodiment shown in FIG. 3 is similar to that shown inFIG. 1, and the description is not repeated.

FIG. 3 shows a digital computer 40 to which are transmitted a speedsignal, an armature current signal, and a cycle indicator signal. Thespeed signal is either the master reference signal or the steadyreference signal, either of which is obtained as already described. Thearmature current signal again is the voltage at shunt 24. Alimit switch42 is operatively associated with the mold to be opened and closed witheach oscillation of 'the mold. I set the computer to determine theaverage magnitude of the armature current during a representative memberof mold oscillation cycles. Operation of the limit switch signals thecomputer as to the number of cycles which take place. When the selectednumber of oscillations have taken place, I termi nate the observation. Ioperate the motor 12 with the mold empty as already described forcalibration purposes, thus enabling the computer tosubtract the portionof the armature current signal utilized in overcoming motor losses andmechanical losses. The computer has a calibration switch 43, a readswitch 44, and a mold friction print out 45.

OPERATION In both embodiments of the invention, the voltage at the shunt2 4 is representative of the load on the motor 10. The factors whichcontribute to this load include the varying forces to lift and lower themold during different portions of its oscillation cycle, motor losses,mechanical losses in the gearing and other mechanisms, and acceleratingforces, beside mold frictions. I measure the voltage at the shunt 24,but I obtain a direct reading of mold friction in this measurement sinceI compensate for these other factors. I compensate for the varyinglifting force by averaging the voltage through a representative numberof cycles, either with an integrating amplifier as shown in FIG. 1 orwith a digital computer as shown in FIG. 3. I compensate for motorlosses and mechanical losses by calibrating the voltmeter 28 whiledriving the mold up and down empty. I compensate for the acceleratingforces by temporarily utilizing a steady reference signal to control thespeed of motor 12 in place of the varying master reference signal.

From the foregoing description, it is seen that my invention affords asimple method and mechanism for determining the magnitude of moldfriction, and can be applied readily to existing casting machineswithout disturbing their structure, as is necessary if load cells areplaced under a mold. The determination is accurate, since compensationis made for all other factors which would produce an erroneousdetermination. The mold friction in force units is readily determinedfrom the motor load measurements when a proportionality factor isdetermined from motor characteristics, gear ratios, and cam designs.

I claim:

1. In a continuous-casting operation in which liquid metal is introducedcontinuously to the top of an openended mold, a partially solidifiedcasting of indefinite length emerges from the bottom of the mold, andthe mold is oscillated vertically, the movement of said casting throughsaid mold being opposed by friction, and in which the drive means foroscillating the mold includes a motor operatively connected therewith,the magnitude of the armature current to said motor being proportionalto the load on the motor, an improved torsfor which compensation is madeinclude the varying force required to lift and lower the mold throughdifferent portions of its oscillation cycle, motor losses, andmechanical losses in the gearing and other mechanisms.

3.A method as defined in claim 2 in which the varying force required tolift and lower the mold is compensated for by averaging the currentmeasurement through said cycles.

4. A method as defined in claim 2 in which the mold losses andmechanical losses are compensated for by running said motor with themold empty and calibrating the measuring means to read zero with themotor driving the empty mold.

5. A method as defined in claim 2 in which an additional factor 'forwhich compensation is made is the accelerating force on the motorbrought about by changes in the speed of mold oscillation, and theaccelerating force is compensated for by operating the motor at aconstant speed during said cycles.

6. In a continuous-casting operation in which liquid metal is introducedcontinuously to the top of an openended mold, a partially solidifiedcasting of indefinite length emerges from the bottom of the mold, andthe mold is oscillated vertically, the movement of said casting throughsaid mold being opposed by friction and in which the drive means foroscillating the mold includes a variable speed d-c motor operativelyconnected therewith, and means for producing a master reference signalfor controlling the casting speed and maintaining the speed at which themold oscillatesat a predetermined ratio to the casting speed, themagnitude of the armature current being proportional to the load on themotor, an improved method of determining the magnitude of the moldfriction, said method comprising measuring the average magnitude of thearmature current to said motor with the motor oscillating the moldthrough a representative number of cycles during a casting operation,calibrating the measuring means to compensate for motor losses andmechanical losses by running said motor with the mold empty and settingthe measuring means to zero, and replacing said master reference signalwith a steady reference signal while the mold friction determination ismade to compensate for accelerating forces.

7. A method as defined in claim 6 in which said steady reference signalapproximately equals the magnitude of said master reference signal atthe beginning of the determination.

8. A method as defined in claim 6 in which the average magnitude of thearmature current is obtained by applying said steady reference signal toan integrating amplifier.

9. A method as defined in claim 6 in which the average magnitude of thearmature current is obtained by applying said steady reference signal toa digital computer through a predetermined number of oscillation cycles.

10. A method as defined in claim 6 in which the mechanical losses are ata maximum at the beginning of a casting operation, but diminish andreach a steady magnitude as casting progresses, and said calibratingstep is conducted immediately after the completion of a castingoperation.

II. In a continuous-casting machine which comprises an open-ended moldfor receiving liquid metal and from the bottom of which a partiallysolidified casting emerges, a motor operatively connected with said moldfor oscillating it vertically, the magnitude of the armature current tosaid motor being proportional to the load on the motor, one of thefactors contributing to said load being the frictional force between themold and the casting, the combination therewith of a mechanism fordetermining the magnitude of the mold friction, said mechanismcomprising; means for measuring the armature current, and means forcompensating for other factors contributing to said load, whereby thearmature current measurement affords a direct determination of moldfriction.

12. A continuous-casting machine as defined in claim 11 in which anotherfactor contributing to said load is the varying force required to liftand lower the mold through different portions of its oscillation cycle,and the means for compensating for variations in the lifting andlowering force includes means for averaging the armature current througha representative number of oscillation cycles.

13. A machine as defined in claim 12 in which said averaging means is adigital computer.

14. A machine as defined in claim 11 in which said averaging means is anintegrating amplifier.

15. A continuous-casting machine as defined in claim 11 in which otherfactors contributing to said load are motor losses and mechanicallosses, and the means for compensating for said losses includecalibration means which are set to show a zero with said motor drivingan empty mold.

16. A continuous-casting machine as defined in claim 11 in which anotherfactor contributing to said load is acceleration force brought about bychanges in the speed at which said mold oscillates. and the means forcompensating for acceleration force includes means for producing asteady reference signal for controlling the armature current.

l7. In a continuous-casting machine which comprises an open-ended moldfor receiving liquid metal and from the bottom of which a partiallysolidified casting emerges, a motor operatively connected with said moldfor oscillating it vertically, and means for producing a varying masterreference signal which controls the speed of said motor, the magnitudeof the armature current to said motor being proportional to the load onsaid motor, the factors contributing to said load including moldfriction, the varying force required to lift and lower the mold throughdifferent portions of its oscillation cycle, motor losses, mechanicallosses, and accelerating forces brought about by speed changes, thecombination therewith of a mechanism for determining the magnitude ofthe mold friction, said mechanism comprising means for measuring thearmature current averaged over a representative number of oscillationcycles to compensate for the varying force required to lift and lowerthe mold, means connected with said measuring means for compensating forthe effect of said losses, and means for replacing said master referencesignal with a steady reference signal while the mold frictiondetermination is made to compensate for said accelerating forces.

18. A mechanism as defined in claim 17 in which the means for measurngthe armature current includes a voltmeter and an integrating amplifieroperatively connected with said voltmeter and receiving said steadyreference signal.

19. A mechanism as defined in claim 17 in which the means for measuringthe armature current includes a digital computer and a print-outoperatively connected with said computer, said computer receiving saidsteady reference signal.

20. A mechanism as defined in claim 17 in which the means for replacingsaid master signal with a steady reference signal includes a blockingrectifier and a potentiometer connected to the circuit which carriessaid signals, said blocking rectifier normally permitting said masterreference signal to pass, but permitting said steady reference signal topass when its magnitude is greater than said master reference signal.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 893502 Dated uly 8 1975 Frank Slamar Invent0r(s) It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 4, line 51 after "voltage" insert a comma.

Column 5, line 29, "read" should read "read" Column 7, line 45, after"zero" insert reading Signed and Scaled this sixteenth D21) OfSeptemberl975 [SEAL] Attest.

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Pare/usand Trademarks

1. In a continuous-casting operation in which liquid metal is introducedcontinuously to the top of an open-ended mold, a partially solidifiedcasting of indefinite length emerges from the bottom of the mold, andthe mold is oscillated vertically, the movement of said casting throughsaid mold being opposed by friction, and in which the drive means foroscillating the mold includes a motor operatively connected therewith,the magnitude of the armature current to said motor being proportionalto the load on the motor, an improved method of determining themagnitude of the mold friction, said method comprising measuring themagnitude of the armature current to said motor with the motoroscillating the mold through a representative number of cycles during acasting operation, and compensating for factors which contribute to theload on the motor other than mold friction.
 2. A method as defined inclaim 1 in which the factors for which compensation is made include thevarying force required to lift and lower the mold through differentportions of its oscillation cycle, motor losses, and mechanical lossesin the gearing and other mechanisms.
 3. A method as defined in claim 2in which the varying force required to lift and lower the mold iscompensated for by averaging the current measurement through saidcycles.
 4. A method as defined in claim 2 in which the mold losses andmechanical losses are compensated for by running said motor with themold empty and calibrating the measuring means to read zero with themotor driving the empty mold.
 5. A method as defined in claim 2 in whichan additional factor for which compensation is made is the acceleratingforce on the motor brought about by changes in the speed of moldoscillation, and the accelerating force is compensated for by operatingthe motor at a constant speed during said cycles.
 6. In acontinuous-casting operation in which liquid metal is introducedcontinuously to the top of an open-ended mold, a partially solidifiedcasting of indefinite length emerges from the bottom of the mold, andthe mold is oscillated vertically, the movement of said casting throughsaid mold being opposed by friction and in which the drive means foroscillating the mold includes a variable speed d-c motor operativelyconnected therewith, and means for producing a master reference sIgnalfor controlling the casting speed and maintaining the speed at which themold oscillates at a predetermined ratio to the casting speed, themagnitude of the armature current being proportional to the load on themotor, an improved method of determining the magnitude of the moldfriction, said method comprising measuring the average magnitude of thearmature current to said motor with the motor oscillating the moldthrough a representative number of cycles during a casting operation,calibrating the measuring means to compensate for motor losses andmechanical losses by running said motor with the mold empty and settingthe measuring means to zero, and replacing said master reference signalwith a steady reference signal while the mold friction determination ismade to compensate for accelerating forces.
 7. A method as defined inclaim 6 in which said steady reference signal approximately equals themagnitude of said master reference signal at the beginning of thedetermination.
 8. A method as defined in claim 6 in which the averagemagnitude of the armature current is obtained by applying said steadyreference signal to an integrating amplifier.
 9. A method as defined inclaim 6 in which the average magnitude of the armature current isobtained by applying said steady reference signal to a digital computerthrough a predetermined number of oscillation cycles.
 10. A method asdefined in claim 6 in which the mechanical losses are at a maximum atthe beginning of a casting operation, but diminish and reach a steadymagnitude as casting progresses, and said calibrating step is conductedimmediately after the completion of a casting operation.
 11. In acontinuous-casting machine which comprises an open-ended mold forreceiving liquid metal and from the bottom of which a partiallysolidified casting emerges, a motor operatively connected with said moldfor oscillating it vertically, the magnitude of the armature current tosaid motor being proportional to the load on the motor, one of thefactors contributing to said load being the frictional force between themold and the casting, the combination therewith of a mechanism fordetermining the magnitude of the mold friction, said mechanismcomprising; means for measuring the armature current, and means forcompensating for other factors contributing to said load, whereby thearmature current measurement affords a direct determination of moldfriction.
 12. A continuous-casting machine as defined in claim 11 inwhich another factor contributing to said load is the varying forcerequired to lift and lower the mold through different portions of itsoscillation cycle, and the means for compensating for variations in thelifting and lowering force includes means for averaging the armaturecurrent through a representative number of oscillation cycles.
 13. Amachine as defined in claim 12 in which said averaging means is adigital computer.
 14. A machine as defined in claim 11 in which saidaveraging means is an integrating amplifier.
 15. A continuous-castingmachine as defined in claim 11 in which other factors contributing tosaid load are motor losses and mechanical losses, and the means forcompensating for said losses include calibration means which are set toshow a zero with said motor driving an empty mold.
 16. Acontinuous-casting machine as defined in claim 11 in which anotherfactor contributing to said load is acceleration force brought about bychanges in the speed at which said mold oscillates, and the means forcompensating for acceleration force includes means for producing asteady reference signal for controlling the armature current.
 17. In acontinuous-casting machine which comprises an open-ended mold forreceiving liquid metal and from the bottom of which a partiallysolidified casting emerges, a motor operatively connected with said moldfor oscillating it vertically, and means for producing a varying masterreference signal which controls the speed of said motor, the magnitudeoF the armature current to said motor being proportional to the load onsaid motor, the factors contributing to said load including moldfriction, the varying force required to lift and lower the mold throughdifferent portions of its oscillation cycle, motor losses, mechanicallosses, and accelerating forces brought about by speed changes, thecombination therewith of a mechanism for determining the magnitude ofthe mold friction, said mechanism comprising means for measuring thearmature current averaged over a representative number of oscillationcycles to compensate for the varying force required to lift and lowerthe mold, means connected with said measuring means for compensating forthe effect of said losses, and means for replacing said master referencesignal with a steady reference signal while the mold frictiondetermination is made to compensate for said accelerating forces.
 18. Amechanism as defined in claim 17 in which the means for measurng thearmature current includes a voltmeter and an integrating amplifieroperatively connected with said voltmeter and receiving said steadyreference signal.
 19. A mechanism as defined in claim 17 in which themeans for measuring the armature current includes a digital computer anda print-out operatively connected with said computer, said computerreceiving said steady reference signal.
 20. A mechanism as defined inclaim 17 in which the means for replacing said master signal with asteady reference signal includes a blocking rectifier and apotentiometer connected to the circuit which carries said signals, saidblocking rectifier normally permitting said master reference signal topass, but permitting said steady reference signal to pass when itsmagnitude is greater than said master reference signal.